1. Field of the Invention
This invention relates to an improved process and apparatus for electrolytic plating of substrates and is more particularly concerned with a process and apparatus for controlling the uniformity of thickness of electroplated coatings of metal on substrates.
2. Description of the Prior Art
In the electroplating of a coating of metal such as copper and the like on a substrate it is desirable and, in many cases, essential that the coating so applied have a uniform thickness throughout all areas thereof. One example of this is found in the fabrication of printed circuit boards. Thus, the current technology for production of a conductive path between two or more layers of a printed circuit board involves the electroless chemical deposition of a thin layer of copper in holes drilled through the board. The thickness of this layer is then increased by the electrolytic deposition of copper. In order to maintain the hole sizes within design parameters it is important that the thickness of the copper deposited by the electroplating be uniform at all locations on the board. After the electroplating has been completed a part of the copper on the external surface of the board is removed by chemical etching while the required copper circuitry, protected by an etch resist, is left in place. If the electrodeposited layer of copper on the board is not uniform in thickness, the width of the lines in the circuit may be reduced during the etching time necessary for complete removal of copper in areas of high thickness.
The uniformity of thickness of coating produced by electrodeposition of a metal such as copper depends largely on achieving uniform current distribution over the cathode. This, in turn, depends to a large degree on the anode-cathode geometry. Methods of producing a uniform cathode current density frequently involve protection of high current density areas by shields placed in the plating solution between the anode and cathode to form a physical barrier to the field; Loch U.S. Pat. No. 4,323,433 shows a typical device of this type. Such shields must be configured to accomodate a particular geometrical arrangement of anode to cathode for best results. Hence, when the size or shape of the cathode changes, a new shield design must be employed.
Another method of preventing high plating thickness at the edges (high current density areas) of a substrate involves the use of an uncoated metal plating rack which surrounds the work load and diverts current flow from the board edge. This method suffers a number of disadvantages. Thus, large amounts of metal are deposited on the rack. This metal has to be stripped from the rack at frequent intervals thereby leading to additional costs as well as wastage of metal and the necessity to remove metal from the stripping solution prior to environmentally sound disposal of the latter waste solution. Further, in order for the device to be effective it is necessary to fill any windows or gaps in the rack. In the case of substrates of varying size and shape, this is frequently difficult to accomplish without resorting to the use of "dummy" substrates, i.e. substrates prepared in a shape to match those of the actual substrates to be plated but having no usefulness other than serving to fill the window. Such a practice is clearly undesirable for obvious reasons.
The present invention provides a method and apparatus for controlling the uniformity of thickness of coatings of metal elecrtrodeposited on substrates. The key to achieving such control lies in the use of a buoyant shield of novel construction which also serves as a rack for holding a series of substrates to be plated. Various types of racks for use in plating baths, including racks provided with buoyancy, have been described in the art. Illustratively, Elsenheimer U.S. Pat. No. 2,362,474 describes a rack for suspending holloware in a plating bath. The rack is provided with flotation material which causes the rack to be rotated through 180.degree. in the vertical direction as it is lowered into the bath and to be rotated back to its original position as it is withdrawn from the bath. Saffel U.S. Pat. No. 2.654,707 and Schaer U.S. Pat. No. 3,901,788 show similar devices.
Fukanuma U.S. Pat. No. 3,658,663 describes a rack for holding articles such as transistor heads so that only a portion of the article (the outer leads in the case of the transistor heads) is immersed in the plating bath.
The present invention provides a device which serves a double purpose by acting as a cathode shield and also as a buoyant rack for holding the substrates to be plated.